As a means for controlling the output of a light source by using an alternating current (AC) power source, a phase control system including a semiconductor switching element has been commonly used. Generally, in the phase control system, the switching element is serially connected between the AC power source and the light source (lighting load). The dimming of the light source is performed by controlling the conduction angle within one cycle of the AC voltage, i.e. the range of angles in which the switching element is turned on, so as to vary the effective voltage supplied from the AC power source to the lighting load.
In one type of conventional dimmer using the phase control system, a zero-crossing point (the point in time at which the AC voltage becomes zero) is used as the reference point for controlling the conduction angle. This type of dimmer includes a zero-crossing detector for detecting the zero-crossing point.
However, the zero-crossing detector may possibly malfunction due to noise contamination or waveform distortion of the AC voltage supplied from the power source; in the case of a dimmer circuit with multiple dimmers connected thereto, the switching noise generated by one dimmer can contaminate the AC voltage supplied to another dimmer circuit, causing the zero-crossing detector to malfunction. In particular, if the AC voltage is supplied from a power generation by natural energy, such as wind power generation or solar power generation, the power is unstable and it is difficult to completely eliminate the noise or waveform distortion even if the power is controlled by the “smart grid”, i.e. a power grid system having the function of autonomously controlling the electric power supply and demand by means of telecommunication devices and computers. Using an in-house power generation is also more likely to cause the noise contamination or waveform distortion than using the commercial power supply. If such a noise or waveform distortion occurs, the zero-crossing detector will mistake a point at which the AC voltage is not actually zero for the zero-crossing point, or fail to detect a point at which the AC voltage is actually zero as the zero-crossing point. As a result, a brightness fluctuation or flicker of the lighting apparatus or the like occurs. Such problems are particularly noticeable in the case of an apparatus using an LED as the light source.
One method for more accurately detecting the zero-crossing point uses a phase lock loop (PLL) circuit (for example, see Patent Document 1). In this system, the zero-crossing detector generates a pulse signal every time it detects the zero-crossing point. The PLL circuit, winch includes an oscillator for generating an oscillating signal, receives the pulse signal and outputs an oscillating signal while performing a feedback control to synchronize the oscillating signal with the pulsed signal (i.e. to make these signals in phase with each other). Pulse signals originating from noise or other factors are out of phase with the oscillating signal and hence can be removed. However, the PLL circuit has the problem that, if the response speed is set at a low level, a considerable length of time will be needed to establish or restore the synchronization when the lighting apparatus is energized or the synchronization is broken for some reasons, during which time the amount of light of the apparatus will fluctuate. On the other hand, setting a higher response speed to avoid this problem will lead to an insufficient removal of the noise.
Patent Document 2 discloses a dimmer in which a fast Fourier transform of an AC signal before being sent to the zero-crossing detector is performed to extract only the fundamental component of the AC signal, and this signal, which is free from noise, is sent to the zero-crossing detector so that the zero-crossing point can be detected on the basis of the noise-free signal. However, this dimmer is expensive since it requires a high-performance computing unit to handle a large number of sampled data of the signal of the AC voltage during the fast Fourier transform operation.